(a) Field of the Invention
The invention relates to an image processing apparatus and a method thereof, particularly to an image processing apparatus and a method to remove the block effect in an image.
(b) Description of the Related Art
During image transmission, generally the image to be transmitted will be compressed to decrease the usage of network bandwidths and MPEG (Moving Picture Experts Group) is one commonly used image compression technique.
FIG. 1 shows a schematic diagram illustrating a local image comprising four macroblocks. During image compression, the MPEG technique treats each 8×8 macroblock as one unit to do compression. During decompression, an 8×8 macroblock 12 is used as one decoded unit to decode. However, the decompressed image sometimes indistinctly shows comprising a plurality of blocks, called “block effect” or “mosaic phenomenon”.
The reason for having the block effect will be described as follows. As the original image is compressed by the MPEG technique, one 8×8 macroblock is used as one coded unit to do compression. Since most of high-frequency information are eliminated during quantization, the compressed macroblock comprises mostly only low-frequency information (direct-current (DC) coefficients such as average luminance). If the compressed macroblocks are decoded to form the image 11, due to certain difference existed between average luminance of two macroblocks, the luminance difference between the pixel 125 and the pixel 124 near the block boundary 13 will be excessively large and thus the block effect appears.
On the other hand, in the process of transmitting the image signal comprising a plurality of programs, due to various reasons, some adjacent pixels of the original image sometimes will be deleted and the number of the deleted rows or columns of pixels for each program will be different. During program switching, the back-end image processing apparatus may not predict the block boundary between adjacent macroblocks in the image correctly to smoothly perform a deblock process. Taking the image 11 shown in FIG. 1 as one example, the image 11 is the upper left local image of a complete image where the pixels of the four columns shown by dashed lines on the farthest left represent the deleted pixels and thus the block boundary 13 disposed between the eighth column and the ninth column in the original image become being disposed between the fourth column and the fifth column in the processed image.
In order to remove the block effect, that is, to have all the block boundaries 13 be blurred, the optimum method is to provide a deblock filter in a MPEG II decoder since the MPEG II decoder receives MPEG data streams that have the related information of the block boundaries and can decode MPEG data streams and correctly determine the boundary location of adjacent blocks. Therefore, the deblock filter can remove the block effect at the correct position.
However, regardless of whether MPEG data streams can be received or not, the above mentioned block effect exists in all of the image processing apparatuses. How to correctly determine the boundary between adjacent blocks even under the condition of being unable to receive MPEG data streams to further remove the block effect is a big challenge for the industry.